2009 Pearson Education, Inc. All rights reserved. 1 11 Object-Oriented Programming: Inheritance.

2009 Pearson Education, Inc. All rights reserved.

1

1111Object-Oriented Programming: Inheritance


2

Say not you know another entirely, till you have divided an inheritance with him.

– Johann Kasper Lavater

This method is to define as the number of a classthe class of all classes similar to the given class.

– Bertrand Russell

Good as it is to inherit a library,it is better to collect one.

– Augustine Birrell

Save base authority from others’ books.– William Shakespeare


3

OBJECTIVES

In this chapter you will learn: How inheritance promotes software reusability. The concepts of base classes and derived classes. To create a derived class that inherits attributes

and behaviors from a base class. To use access modifier protected to give derived-class

methods access to base-class members. To access base-class members with base. How constructors are used in inheritance hierarchies. The methods of class object, the direct or

indirect base class of all classes.


4

11.1 Introduction

11.2 Base Classes and Derived Classes

11.3 protected Members

11.4 Relationship between Base Classes and Derived Classes

11.5 Constructors in Derived Classes

11.6 Software Engineering with Inheritance

11.7 Class object


5

11.1 Introduction

• Inheritance allows a new class to absorb an existing class’s members.

• A derived class normally adds its own fields and methods to represent a more specialized group of objects.

• Inheritance saves time by reusing proven and debugged high-quality software.


6

11.1 Introduction (Cont.)

• The direct base class is the base class which the derived class explicitly inherits.

• An indirect base class is any class above the direct base class in the class hierarchy.

• The class hierarchy begins with class object.


7

11.1 Introduction (Cont.)

• The is-a relationship represents inheritance.

• For example, a car is a vehicle, and a truck is a vehicle.

• New classes can inherit from thousands of pre-built classes in class libraries.


8

11.2 Base Classes and Derived Classes

• Figure 11.1 lists several simple examples of base classes and derived classes.

• Note that base classes are “more general,” and derived classes are “more specific.”

Base class Derived classes

Student GraduateStudent, UndergraduateStudent

Shape Circle, Triangle, Rectangle

Loan CarLoan, HomeImprovementLoan, MortgageLoan

Employee Faculty, Staff, HourlyWorker, CommissionWorker

BankAccount CheckingAccount, SavingsAccount

Fig. 11.1 | Inheritance examples.


9

11.2 Base Classes and Derived Classes (Cont.)

• The UML class diagram of Fig. 11.2 shows an inheritance hierarchy representing a university community.

• Each arrow represents an is-a relationship.

Fig. 11.2 | UML class diagram showing an inheritance hierarchyfor university CommunityMembers.


10


• Now consider the Shape inheritance hierarchy in Fig. 11.3.

• We can follow the arrows to identify several is-a relationships.

Fig. 11.3 | UML class diagram showing an inheritance hierarchy for Shapes.


11


• Objects of all classes that extend a common base class can be treated as objects of that base class.

• However, base-class objects cannot be treated as objects of their derived classes.

• When a derived class needs a customized version of an inherited method, the derived class can override the base-class method.


12

11.3 protected Members

• A base class’s private members are inherited by derived classes, but are not directly accessible by derived-class methods and properties.

• A base class’s protected members can be accessed by members of that base class and by members of its derived classes.

• A base class’s protected internal members can be accessed by members of a base class, the derived classes and by any class in the same assembly.


13

11.3 protected Members (Cont.)

Software Engineering Observation 11.1Properties and methods of a derived class cannot directlyaccess private members of the base class. A derived class can change the state of private base-class fields only through non-private methods and properties provided in the base class.

Software Engineering Observation 11.2

If a derived class can access its base class’s private fields,classes that inherit from that base class could access thefields as well. This would propagate access to what shouldbe private fields, and the benefits of informationhiding would be lost.


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1 // Fig. 11.4: CommissionEmployee.cs

2 // CommissionEmployee class represents a commission employee.

3 public class CommissionEmployee : object

4 {

5 private string firstName;

6 private string lastName;

7 private string socialSecurityNumber;

8 private decimal grossSales; // gross weekly sales

9 private decimal commissionRate; // commission percentage

10

11 // five-parameter constructor

12 public CommissionEmployee( string first, string last, string ssn,

13 decimal sales, decimal rate )

14 {

15 // implicit call to object constructor occurs here

Fig. 11.4 | CommissionEmployee class represents acommission employee. (Part 1 of 5.)

Outline

CommissionEmployee.cs

(1 of 5 )

• CommissionEmployee (Fig. 11.4) represents anemployee who is paid a percentage of their sales.

A colon (:) followed a class name at the end of the class declaration header indicates that the class extends the class to the right of the colon.


15

16 firstName = first;

17 lastName = last;

18 socialSecurityNumber = ssn;

19 GrossSales = sales; // validate gross sales via property

20 CommissionRate = rate; // validate commission rate via property

21 } // end five-parameter CommissionEmployee constructor

22

23 // read-only property that gets commission employee's first name

24 public string FirstName

25 {

26 get

27 {

28 return firstName;

29 } // end get

30 } // end property FirstName

31


Outline


(2 of 5 )


16

32 // read-only property that gets commission employee's last name

33 public string LastName

34 {

35 get

36 {

37 return lastName;

38 } // end get

39 } // end property LastName

40

41 // read-only property that gets

42 // commission employee's social security number

43 public string SocialSecurityNumber

44 {


Outline


(3 of 5 )


17

45 get

46 {

47 return socialSecurityNumber;

48 } // end get

49 } // end property SocialSecurityNumber

50

51 // property that gets and sets commission employee's gross sales

52 public decimal GrossSales

53 {

54 get

55 {

56 return grossSales;

57 } // end get

58 set

59 {

60 grossSales = ( value < 0 ) ? 0 : value;

61 } // end set

62 } // end property GrossSales

63

64 // property that gets and sets commission employee's commission rate

65 public decimal CommissionRate

66 {

67 get


Outline


(4 of 5 )


18

68 {

69 return commissionRate;

70 } // end get

71 set

72 {

73 commissionRate = ( value > 0 && value < 1 ) ? value : 0;

74 } // end set

75 } // end property CommissionRate

76

77 // calculate commission employee's pay

78 public decimal Earnings()

79 {

80 return commissionRate * grossSales;

81 } // end method Earnings

82

83 // return string representation of CommissionEmployee object

84 public override string ToString()

85 {

86 return string.Format(

87 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}",

88 "commission employee", FirstName, LastName,

89 "social security number", SocialSecurityNumber,

90 "gross sales", GrossSales, "commission rate", CommissionRate );

91 } // end method ToString

92 } // end class CommissionEmployee


Outline


(5 of 5 )


19

11.4 Relationship between Base Classes and Derived Classes (Cont.)

• A colon (:) followed a class name at the end of the class declaration header indicates that the class extends the classto the right of the colon.

• Every C# class directly or indirectly inherits object’s methods.

• If a class does not specify that it inherits from another class,it implicitly inherits from object.


The compiler sets the base class of a class to object when the class declaration does not explicitly extend a base class.


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• Declaring instance variables as private and providing public properties to manipulate and validate them helps enforce good software engineering.

• Constructors are not inherited.

• Either explicitly or implicitly, a call to the base-class constructor is made.

• Class object’s default (empty) constructor does nothing.

• Note that even if a class does not have constructors, the default constructor will call the base class’s default or parameterless constructor.


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• Method ToString is special—it is one of the methods that every class inherits directly or indirectly from class object.

• Method ToString returns a string representing an object.

• Class object’s ToString method is primarily a placeholder that typically should be overridden by a derived class.

• To override a base-class method, a derived class must declare a method with keyword override.

• The method must have the same signature (method name, number of parameters and parameter types) and return type as the base-class method.


22


Common Programming Error 11.1It is a compilation error to override a method with a different access modifier. If a public method could be overridden as a protected or private method, the derived-class objects would not respond to the same method calls as base-class objects.


23

1 // Fig. 11.5: CommissionEmployeeTest.cs

2 // Testing class CommissionEmployee.

3 using System;

4

5 public class CommissionEmployeeTest

6 {

7 public static void Main( string[] args )

8 {

9 // instantiate CommissionEmployee object

10 CommissionEmployee employee = new CommissionEmployee( "Sue",

11 "Jones", "222-22-2222", 10000.00M, .06M );

12

13 // display commission-employee data

14 Console.WriteLine(

15 "Employee information obtained by properties and methods: \n" );

16 Console.WriteLine( "First name is {0}", employee.FirstName );

17 Console.WriteLine( "Last name is {0}", employee.LastName );

18 Console.WriteLine( "Social security number is {0}",

• Figure 11.5 tests class CommissionEmployee.

Fig. 11.5 | Testing class CommissionEmployee. (Part 1 of 3.)

Outline

CommissionEmployeeTest.cs

(1 of 3 )


24

19 employee.SocialSecurityNumber );

20 Console.WriteLine( "Gross sales are {0:C}", employee.GrossSales );

21 Console.WriteLine( "Commission rate is {0:F2}",

22 employee.CommissionRate );

23 Console.WriteLine( "Earnings are {0:C}", employee.Earnings() );

24

25 employee.GrossSales = 5000.00M; // set gross sales

26 employee.CommissionRate = .1M; // set commission rate

27

28 Console.WriteLine( "\n{0}:\n\n{1}",

29 "Updated employee information obtained by ToString", employee );

30 Console.WriteLine( "earnings: {0:C}", employee.Earnings() );

31 } // end Main

32 } // end class CommissionEmployeeTest


Outline


(2 of 3 )


25

Employee information obtained by properties and methods:

First name is Sue

Last name is Jones

Social security number is 222-22-2222

Gross sales are $10,000.00

Commission rate is 0.06

Earnings are $600.00

Updated employee information obtained by ToString:

commission employee: Sue Jones

social security number: 222-22-2222

gross sales: $5,000.00

commission rate: 0.10

earnings: $500.00


Outline


(3 of 3 )


26

1 // Fig. 11.6: BasePlusCommissionEmployee.cs

2 // BasePlusCommissionEmployee class represents an employee that receives

3 // a base salary in addition to a commission.

4 public class BasePlusCommissionEmployee

5 {






11 private decimal baseSalary; // base salary per week

12

13 // six-parameter constructor

14 public BasePlusCommissionEmployee( string first, string last,

15 string ssn, decimal sales, decimal rate, decimal salary )

16 {

• We now declare and test a separate class BasePlusCommissionEmployee (Fig. 11.6),

Fig. 11.6 | BasePlusCommissionEmployee class represents an employeethat receives a base salary in addition to a commission. (Part 1 of 6.)

Outline

BasePlusCommissionEmployee.cs

(1 of 6 )


27



19 lastName = last;




23 BaseSalary = salary; // validate base salary via property

24 } // end six-parameter BasePlusCommissionEmployee constructor

25


27 // base-salaried commission employee's first name


29 {

30 get

31 {


33 } // end get


35


Outline


(2 of 6 )



37 // base-salaried commission employee's last name


39 {

40 get

41 {

42 return lastName;

43 } // end get


45


47 // base-salaried commission employee's social security number


49 {

50 get

51 {


53 } // end get


55

56 // property that gets and sets

57 // base-salaried commission employee's gross sales


59 {

60 get


Outline


(3 of 6 )


29

61 {


63 } // end get

64 set

65 {


67 } // end set


69


71 // base-salaried commission employee's commission rate


73 {

74 get

75 {


77 } // end get

78 set

79 {



Outline


(4 of 6 )


30

81 } // end set


83


85 // base-salaried commission employee's base salary

86 public decimal BaseSalary

87 {

88 get

89 {

90 return baseSalary;

91 } // end get

92 set

93 {


Outline


(5 of 6 )


31

94 baseSalary = ( value < 0 ) ? 0 : value;

95 } // end set

96 } // end property BaseSalary

97

98 // calculate earnings

99 public decimal Earnings()

100 {

101 return BaseSalary + ( CommissionRate * GrossSales );

102 } // end method earnings

103

104 // return string representation of BasePlusCommissionEmployee


106 {


108 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}\n{9}: {10:C}",

109 "base-salaried commission employee", FirstName, LastName,


111 "gross sales", GrossSales, "commission rate", CommissionRate,

112 "base salary", BaseSalary );


114 } // end class BasePlusCommissionEmployee


Outline


(6 of 6 )


32


• Note the similarity between this class and class Commission Employee (Fig. 11.4)—in this example, we do not yet exploit that similarity.


33

1 // Fig. 11.7: BasePlusCommissionEmployeeTest.cs

2 // Testing class BasePlusCommissionEmployee.

3 using System;

4

5 public class BasePlusCommissionEmployeeTest

6 {


8 {

9 // instantiate BasePlusCommissionEmployee object

10 BasePlusCommissionEmployee employee =

11 new BasePlusCommissionEmployee( "Bob", "Lewis",

12 "333-33-3333", 5000.00M, .04M, 300.00M );

13

14 // display base-salaried commission-employee data





• Figure 11.7 tests class BasePlusCommissionEmployee.

Fig. 11.7 | Testing class BasePlusCommissionEmployee. (Part 1 of 3.)

Outline

BasePlusCommissionEmployeeTest.cs

(1 of 3 )


34







25 Console.WriteLine( "Base salary is {0:C}", employee.BaseSalary );

26

27 employee.BaseSalary = 1000.00M; // set base salary

28




32 } // end Main

33 } // end class BasePlusCommissionEmployeeTest


Outline


(2 of 3 )


35

Employee information obtained by properties and methods: First name is Bob Last name is Lewis Social security number is 333-33-3333 Gross sales are $5,000.00 Commission rate is 0.04 Earnings are $500.00 Base salary is $300.00 Updated employee information obtained by ToString: base-salaried commission employee: Bob Lewis social security number: 333-33-3333 gross sales: $5,000.00 commission rate: 0.04 base salary: $1,000.00 earnings: $1,200.00


Outline


(3 of 3 )


36


• Much of the code for BasePlusCommissionEmployeeis similar to the code for CommissionEmployee.

Error-Prevention Tip 11.1 Copying and pasting code from one class to another can spread errors across multiple source-code files. Use inheritance rather than the “copy-and-paste” approach.

Software Engineering Observation 11.4 With inheritance, the common members of all the classes in the hierarchy are declared in a base class. When changes are required for these common features, you need to make the changes only in the base class—derived classes then inherit the changes.


37


2 // BasePlusCommissionEmployee inherits from class CommissionEmployee.

3 public class BasePlusCommissionEmployee : CommissionEmployee

4 {


6

7 // six-parameter derived-class constructor

8 // with call to base class CommissionEmployee constructor



11 : base( first, last, ssn, sales, rate )

12 {



15

• Now we declare class BasePlusCommissionEmployee (Fig. 11.8), which extends class CommissionEmployee (Fig. 11.4).

Fig. 11.8 | BasePlusCommissionEmployee inherits from classCommissionEmployee. (Part 1 of 3.)

Outline


(1 of 3 )

Class BasePlusCommissionEmployeehas an additional instance variable baseSalary.

Invoke the CommissionEmployee’s five-parameter constructor using a constructor initializer.


38




19 {

20 get

21 {


23 } // end get

24 set

25 {


27 } // end set


29


31 public override decimal Earnings()

32 {


Outline


(2 of 3 )


39

33 // not allowed: commissionRate and grossSales private in base class

34 return baseSalary + ( commissionRate * grossSales );


36



39 {

40 // not allowed: attempts to access private base-class members


42 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}\n{9}: {10:C}",

43 "base-salaried commission employee", firstName, lastName,

44 "social security number", socialSecurityNumber,

45 "gross sales", grossSales, "commission rate", commissionRate,

46 "base salary", baseSalary );




Outline


(3 of 3 )


40


• A BasePlusCommissionEmployee object is a CommissionEmployee.

• A constructor initializer with keyword base invokes the base-class constructor.

Common Programming Error 11.2A compilation error occurs if a derived-class constructor calls one of its base-class constructors with arguments that do not match the number and types of parameters specified in one of the base-class constructor declarations.


41


• The virtual and abstract keywords indicate that a base-class method can be overridden in derived classes.

• The override modifier declares that a derived-class method overrides a virtual or abstract base-class method.

• This modifier also implicitly declares the derived-class method virtual.

• We need to declare CommissionEmployee’s Earnings method virtual.


42


• The compiler generates additional errors because base class CommissionEmployee’s instance variables are private.

• The errors can be prevented by using the public properties inherited from class CommissionEmployee.

Fig. 11.9 | Compilation errors generated by BasePlusCommissionEmployee (Fig. 11.8) after declaring the Earnings method in Fig. 11.4 with keyword virtual.


43


2 // CommissionEmployee with protected instance variables.

3 public class CommissionEmployee

4 {

5 protected string firstName;

6 protected string lastName;

7 protected string socialSecurityNumber;

8 protected decimal grossSales; // gross weekly sales

9 protected decimal commissionRate; // commission percentage

10




14 {

• Class CommissionEmployee (Fig. 11.10) is modified to declare its instance variables as protected rather than private (Fig. 11.10).

Fig. 11.10 | CommissionEmployee with protected instance variables. (Part 1 of 5.)

Outline


( 1 of 5 )


44



17 lastName = last;





22



25 {

26 get

27 {


29 } // end get


31



34 {

35 get

36 {


Outline


( 2 of 5 )


45

37 return lastName;

38 } // end get


40




44 {

45 get

46 {


48 } // end get


50



53 {

54 get

55 {


57 } // end get

58 set

59 {


Outline


( 3 of 5 )



61 } // end set


63



66 {

67 get

68 {


70 } // end get

71 set

72 {


74 } // end set


76


78 public virtual decimal Earnings()

79 {

80 return commissionRate * grossSales;


82



85 {


Outline


( 4 of 5 )


47


87 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}",

88 "commission employee", firstName, lastName,


90 "gross sales", grossSales, "commission rate", commissionRate );




• We also declare the Earnings method virtual in line 78 so that BasePlusCommissionEmployee can override the method.

Outline


( 5 of 5 )


48


2 // BasePlusCommissionEmployee inherits from CommissionEmployee and has

3 // access to CommissionEmployee's protected members.

4 public class BasePlusCommissionEmployee : CommissionEmployee2

5 {


7






13 {



16

• Class BasePlusCommissionEmployee (Fig. 11.11) is modified to extend CommissionEmployee.

• The instance variables are now protected members,so the compiler does not generate errors.

Fig. 11.11 | BasePlusCommissionEmployee inherits from CommissionEmployeeand has access to CommissionEmployee's protected members. (Part 1 of 3.)

Outline


( 1 of 3 )

BasePlusCommissionEmployee’s six-parameter constructor calls class CommissionEmployee’s five-parameter constructor with a constructor initializer.


49




20 {

21 get

22 {


24 } // end get

25 set

26 {


28 } // end set


30



33 {

34 return baseSalary + ( commissionRate * grossSales );



Outline


( 2 of 3 )


50

36



39 {


41 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}\n{9}: {10:C}",

42 "base-salaried commission employee", firstName, lastName,


44 "gross sales", grossSales, "commission rate", commissionRate,

45 "base salary", baseSalary );




Outline


( 3 of 3 )


51



3 using System;

4

5 public class BasePlusCommissionEmployee

6 {


8 {

9 // instantiate BasePlusCommissionEmployee object

10 BasePlusCommissionEmployee basePlusCommissionEmployee =


12 "333-33-3333", 5000.00M, .04M, 300.00M );

13




• Figure 11.12 tests a BasePlusCommissionEmployee object.

• While the output is identical, there is less code repetition and overall this is a better implementation.


Outline


( 1 of 3 )


52

17 Console.WriteLine( "First name is {0}",

18 basePlusCommissionEmployee.FirstName );

19 Console.WriteLine( "Last name is {0}",

20 basePlusCommissionEmployee.LastName );


22 basePlusCommissionEmployee.SocialSecurityNumber );

23 Console.WriteLine( "Gross sales are {0:C}",

24 basePlusCommissionEmployee.GrossSales );


26 basePlusCommissionEmployee.CommissionRate );

27 Console.WriteLine( "Earnings are {0:C}",

28 basePlusCommissionEmployee.Earnings() );

29 Console.WriteLine( "Base salary is {0:C}",

30 basePlusCommissionEmployee.BaseSalary );

31

32 basePlusCommissionEmployee.BaseSalary = 1000.00M; // set base salary

33


35 "Updated employee information obtained by ToString",

36 basePlusCommissionEmployee );

37 Console.WriteLine( "earnings: {0:C}",

38 basePlusCommissionEmployee.Earnings() );

39 } // end Main



Outline


( 2 of 3 )


53

Employee information obtained by properties and methods: First name is Bob Last name is Lewis Social security number is 333-33-3333 Gross sales are $5,000.00 Commission rate is 0.04 Earnings are $500.00 Base salary is $300.00 Updated employee information obtained by ToString: base-salaried commission employee: Bob Lewis social security number: 333-33-3333 gross sales: $5,000.00 commission rate: 0.04 base salary: $1,000.00 earnings: $1,200.00


Outline


( 3 of 3 )


54


Software Engineering Observation 11.5Declaring base-class instance variables private enables the base-class implementation of these instance variables to change without affecting derived-class implementations.

• Using protected instance variables creates several potential problems.

– The derived-class object can set an inherited variable’s value directly without validity checking.

– Derived-class methods would need to be written to depend on the base class’s data implementation.

• You should be able to change the base-class implementation while still providing the same services to the derived classes.


55




4 {






10




14 {

• Class CommissionEmployee (Fig. 11.13) is modified to declare private instance variables and provide public properties.

Fig. 11.13 | CommissionEmployee class represents a commission employee. (Part 1 of 5.)

Outline


( 1 of 5 )




17 lastName = last;





22



25 {

26 get

27 {


29 } // end get


31



34 {

35 get

36 {

37 return lastName;

38 } // end get


40


Outline


( 2 of 5 )


57




44 {

45 get

46 {


48 } // end get


50



53 {

54 get

55 {


57 } // end get

58 set

59 {


61 } // end set



Outline


( 3 of 5 )


58

63



66 {

67 get

68 {


70 } // end get

71 set

72 {


74 } // end set


76



79 {

80 return CommissionRate * GrossSales;



Outline


( 4 of 5 )

Use the class’s properties to obtain the values of its instance variables.


59

82



85 {


87 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}",







Outline


( 5 of 5 )


60


2 // BasePlusCommissionEmployee inherits from CommissionEmployee and has

3 // access to CommissionEmployee's private data via

4 // its public properties.


6 {


8

9 // six-parameter derived class constructor





14 {



• Class BasePlusCommissionEmployee (Fig. 11.14) has several changes to its method implementations.

Fig. 11.14 | BasePlusCommissionEmployee inherits from CommissionEmployeeand has access to CommissionEmployee's private data via its public

properties. (Part 1 of 3.)

Outline


( 1 of 3 )


61

17




21 {

22 get

23 {


25 } // end get

26 set

27 {


29 } // end set




Outline


( 2 of 3 )


62

31



34 {

35 return BaseSalary + base.Earnings();


37



40 {

41 return string.Format( "base-salaried {0}\nbase salary: {1:C}",

42 base.ToString(), BaseSalary );





Outline


( 3 of 3 )

Use CommissionEmployee’s Earnings method to calculate the commission pay, and add it to the BaseSalary.


63


Common Programming Error 11.3 When a base-class method is overridden in a derived class, the derived-class version often calls the base-class version to do a portion of the work. Failure to prefix the base-class method name with the keyword base when referencing the base class’s method causes the derived-class method to call itself.

Common Programming Error 11.4

The use of “chained” base references to refer to amember several levels up the hierarchy—as in base.base.Earnings()—is a compilation error.


64

1 // Fig. 11.15: BasePlusCommissionEmployeeTest.cs


3 using System;

4

5 public class BasePlusCommissionEmployeeTest

6 {


8 {

9 // instantiate BasePlusCommissionEmployee4 object

10 BasePlusCommissionEmployee employee =


12 "333-33-3333", 5000.00M, .04M, 300.00M );

13




• Figure 11.15 performs the same manipulations on a BasePlusCommissionEmployee object.

• By using inheritance and properties, we have efficiently and effectively constructed a well-engineered class.

Fig. 11.15 | Testing class BasePlusCommissionEmployee4. (Part 1 of 3.)

Outline


( 1 of 3 )


65









25 Console.WriteLine( "Base salary is {0:C}", employee.BaseSalary );

26

27 employee.BaseSalary = 1000.00M; // set base salary

28




32 } // end Main

33 } // end class BasePlusCommissionEmployeeTest


Outline


( 2 of 3 )


66

Employee information obtained by properties and methods:

First name is Bob

Last name is Lewis

Social security number is 333-33-3333

Gross sales are $5,000.00

Commission rate is 0.04

Earnings are $500.00

Base salary is $300.00

Updated employee information obtained by ToString:

base-salaried commission employee: Bob Lewis




base salary: $1,000.00

earnings: $1,200.00


Outline


( 3 of 3 )


67

11.5 Constructors in Derived Classes

• The derived-class constructor, before performing its own tasks, invokes its direct base class’s constructor.

• This is done either explicitly or implicitly.

• If the base class is derived from another class, the base-class constructor invokes the constructor of the next class up in the hierarchy, and so on.


68

11.5 Constructors in Derived Classes (Cont.)

Software Engineering Observation 11.6When an application creates a derived-class object, the derived-class constructor calls the base-class constructor (explicitly, via base, or implicitly). The base-class constructor’s body executes to initialize the base class’s instance variables that are part of the derived-class object, then the derived class constructor’s body executes to initialize the derived-class-only instance variables. Even if a constructor does not assign a value to an instance variable, the variableis still initialized to its default value.


69



3 using System;

4


6 {






12




16 {

• Class CommissionEmployee (Fig. 11.16) is modifiedto output text when its constructor is invoked.


Outline


( 1 of 5 )


70



19 lastName = last;




23

24 Console.WriteLine( "\nCommissionEmployee constructor:\n" + this );


26



29 {

30 get

31 {


33 } // end get


35



38 {


Outline


( 2 of 5 )


71

39 get

40 {

41 return lastName;

42 } // end get


44




48 {

49 get

50 {


52 } // end get


54



57 {

58 get

59 {


Outline


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72


61 } // end get

62 set

63 {


65 } // end set


67



70 {

71 get

72 {


74 } // end get

75 set

76 {


78 } // end set


80



83 {


Outline


( 4 of 5 )


73

84 return CommissionRate * GrossSales;


86



89 {


91 "{0}: {1} {2}\n{3}: {4}\n{5}: {6:C}\n{7}: {8:F2}",







Outline


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2 // BasePlusCommissionEmployee class declaration.

3 using System;

4


6 {


8






14 {


• Class BasePlusCommissionEmployee (Fig. 11.17) is modified to output text when its constructor is invoked.

Fig. 11.17 | BasePlusCommissionEmployee class declaration. (Part 1 of 3.)

Outline


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16


18 "\nBasePlusCommissionEmployee5 constructor:\n" + this );


20




24 {

25 get

26 {


28 } // end get

29 set

30 {


32 } // end set


34


Outline


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37 {

38 return BaseSalary + base.Earnings();


40



43 {

44 return string.Format( "base-salaried {0}\nbase salary: {1:C}",

45 base.ToString(), BaseSalary );




Outline


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1 // Fig. 10.18: ConstructorTest.cs

2 // Display order in which base-class and derived-class constructors

3 // are called.

4 using System;

5

6 public class ConstructorTest

7 {


9 {

10 CommissionEmployee employee1 = new CommissionEmployee( "Bob",

11 "Lewis", "333-33-3333", 5000.00M, .04M );

12

13 Console.WriteLine();

14 BasePlusCommissionEmployee employee2 =

15 new BasePlusCommissionEmployee( "Lisa", "Jones",

• Figure 11.18 demonstrates the order in which constructors are called in an inheritance hierarchy.

Fig. 11.18 | Display order in which base-class and derived-classconstructors are called. (Part 1 of 3.)

Outline

ConstructorTest.cs

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16 "555-55-5555", 2000.00M, .06M, 800.00M );

17

18 Console.WriteLine();

19 BasePlusCommissionEmployee employee3 =

20 new BasePlusCommissionEmployee( "Mark", "Sands",

21 "888-88-8888", 8000.00M, .15M, 2000.00M );

22 } // end Main

23 } // end class ConstructorTest

CommissionEmployee constructor:

commission employee: Bob Lewis





base-salaried commission employee: Lisa Jones




base salary: $0.00

(continued on next page...)

Fig. 11.18 | Display order in which base-class and derived-class

constructors are called. (Part 2 of 3.)

Outline

ConstructorTest.cs

( 2 of 3 )


79

(continued from previous page…)

BasePlusCommissionEmployee constructor:

base-salaried commission employee: Lisa Jones




base salary: $800.00


base-salaried commission employee: Mark Sands




base salary: $0.00

BasePlusCommissionEmployee constructor:

base-salaried commission employee: Mark Sands




base salary: $2,000.00

Fig. 11.18 | Display order in which base-class and derived-classconstructors are called. (Part 3 of 3.)

Outline

ConstructorTest.cs

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80

11.6 Software Engineering with Inheritance

• When a new class extends an existing class, the new class inherits the members of the existing class.

• We can customize the new class to meet our needs by including additional members and by overriding base-class members.

• Independent software vendors (ISVs) can develop and sell proprietary classes.

• Users then can derive new classes from these library classes without accessing the source code.


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11.6 Software Engineering with Inheritance (Cont.)


Although inheriting from a class does not require access to the class’s source code, developers often insist on seeing the source code to understand how the class is implemented. They may, for example, want to ensure that they are extending a class that performs well and is implemented securely.

• Effective software reuse greatly improves the software-development process.

• Object-oriented programming facilitates software reuse.

• The availability of class libraries delivers the maxi mum benefits of software reuse.


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At the design stage in an object-oriented system, the designer often finds that certain classes are closely related. The designer should “factor out” common members and place them in a base class.


Declaring a derived class does not affect its base class’s source code. Inheritance preserves the in tegrity of thebase class.


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Designers of object-oriented systems should avoid class proliferation. Such proliferation creates management problems and can hinder software reusability, because in a huge class library it becomes difficult for a client to locatethe most appropriate classes.

Performance Tip 11.1

If derived classes are larger than they need to be(i.e., contain too much functionality), memory and pro cessing resources might be wasted. Extend the base class containing the functionality that is closest to what is needed.


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11.7 Class object

• All classes inherit directly or indirectly from the object class.

• Figure 11.19 summarizes object’s methods.

Method Description

Equals This method compares two objects for equality and returns true if they are equal and false otherwise.

Finalize Finalize is called by the garbage collector before it reclaims an object’s memory.

GetHashCode The hashcode value returned can be used by a hashtable to determine the location at which to insert the corresponding value.

Fig. 11.19 | object methods that are inherited directly or indirectlyby all classes. (Part 1 of 2.)


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Method Description

GetType Returns an object of class Type that contains information about the object’s type.

MemberwiseClone This protected method makes a copy of the object on which it is called. Instance-variable values in one object are copied into another object of the same type. For reference types, only the references are copied.

ReferenceEquals This static method returns true if two objects are the same instance or if they are null references.

ToString Returns a string representation of an object. The default implementation returns the namespace and class name.

Fig. 11.19 | object methods that are inherited directly or indirectlyby all classes. (Part 2 of 2.)

11.7 Class object (Cont.)

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2009 Pearson Education, Inc. All rights reserved. 1 11 Object-Oriented Programming: Inheritance.

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